The invention relates to a cathode-ray tube comprising, in an evacuated envelope, an electron gun for generating an electron beam directed onto a target, the electron gun being comprised successively of a cathode centred around an axis, a first grid and a second grid, the first grid forming with the second grid a non-rotationally symmetrical electron lens, mainly a quadrupole lens.
Such a cathode-ray tube may be used for displaying television pictures or in an oscilloscope. In that case the target is a display screen having a phosphor layer or a pattern of phosphor elements luminescing in different colours. Such a tube may also be used for recording pictures. In that case the target is a photoconductive layer. In all such applications it is desirable to have a spot of certain dimensions and without a surrounding haze.
Such a cathode-ray tube is disclosed in U.S. Pat. No. 3,497,673, Hasker. The first grid of the electron gun of the tube there described is provided with an elongated aperture which, in cooperation with the second grid, forms a non-rotationally symmetrical electron lens, which asymmetry is expressed mainly in a quadrupole lens action. After deflection of the electron beam, the spot obtained with such a gun, generally has, in the corners and at the edge of the target, a considerable haze partially or entirely surrounding the spot. The haze produces a region of low but disturbing light intensity near the spot.